One procedure for doping silicon materials with impurities to produce a semiconductor is to bombard silicon wafers with an ion beam. Controlled concentrations of ions strike the wafer and dope the wafer to a particular impurity concentration. Systems for ion doping of semiconductor materials are described in U.S. Pat. Nos. 4,228,358 to Ryding and 4,234,797, also to Ryding. The disclosure of these prior art patents are incorporated herein by reference.
The systems disclosed in these patents employ specifically constructed ion doping chambers. Controlled intensity ion beams are generated and directed to impinge upon a spinning support structure carrying a plurality of wafers about its circumference. By controlled movements of the wafer support, controlled ion doping concentrations of the wafers are achieved.
As described in the '358 patent to Ryding, the prior art systems include a loading station where semiconductor wafers are mounted to a disk-like support for transfer to the ion implantation chamber. Once a specified concentration of doping has taken place, the disk-like support is returned to the loading-unloading station and those wafers already doped are unloaded to make room on the wafer support for undoped wafers.
In these doping systems kinetic energy is transferred from the ion beam to the wafer in the form of heat. If this heat is dissipated during ion implantation desired doping levels can be achieved at a greater rate. One way to dissipate heat build up in the wafers is to make sure the wafers contact the wafer support across the entire wafer surface. Two prior art patents addressing the heat dissipation problem are U.S. Pat. Nos. 4,139,051 to Jones et al. and 4,403,567 to daCosta et al. These two patents disclose a semiconductor material mounting procedure where adhesion between the material and a support is maintained by means of a vacuum.
Experience with prior art vacuum hold down procedures, however, has resulted in some difficulty in transferring semiconductor wafers to and from their support. In particular, nonuniform pressure conditions on the surface of the wafer can lead to breaking of the wafer. This can be caused either through excessive forces exerted on the semiconductor wafer or, nonuniform action of the pressure as the wafer is being removed from the support.